The invention relates to a system for regulating pressure in a pressure chamber, in particular in a pneumatic brake apparatus for a web-type recording medium, in which an actual pressure in the pressure chamber is adjusted dependent on an actual angle of rotation of a rotary valve.
A system of this sort is known from the reference WO 94/27 193. The actual pressure in the pressure chamber is a partial vacuum in relation to the air pressure outside the pressure chamber. Via suction ducts in the surface of the pressure chamber, the partial vacuum suctions the recording medium on which a print image is printed. By means of the suction, and a high frictional force connected therewith between the recording medium and the surface of the pressure chamber, a braking of a transport motion of the recording medium takes place. A pressure sensor arranged in the pressure chamber acquires the actual pressure, which is compared with a predetermined target pressure by means of a control unit, whereby a pressure deviation is determined. Dependent on the pressure deviation, the control unit controls a directcurrent motor for the adjustment of the angle of rotation of the rotary valve with defined voltage impulses in order to reduce or completely remove the pressure deviation.
If a given actual angle of rotation is set at the rotary valve, e.g. by rotating a rotating piston in the valve by the actual angle of rotation on the basis of a reference setting, the pressure in the pressure chamber changes gradually due to pressure compensation processes, until finally a pressure corresponding to the angle of rotation is built up in the pressure chamber. The pressure build-up in the pressure chamber thereby takes place relatively sluggishly. The control unit can determine the presence of a pressure deviation only at a late time, and drives the direct-current motor with a considerable time delay, in order to remove the pressure deviation. The sluggishness of the pressure build-up thus retards the regulation to the target pressure.
This sluggishness is particularly disadvantageous if the frictional relationships in the rotary valve change dependent on time, so that given a driving of the direct-current motor with the defined voltage impulses, an angle of rotation not corresponding to these voltage impulses arises at the control valve, whereby a pressure deviation results that becomes greater as the wear on the rotary valve increases. In addition, the achieved angle of rotation of the rotary valve can differ given identical driving of the direct-current motor, due to tolerance-dependent differences in the direct-current motors. Pressure deviations likewise result from this, which are determined only with a delay by the control unit, with the aid of the pressure sensor.
The sluggishness of the controlling has the result that the actual pressure in the pressure chamber can deviate from the target pressure over a long period of time. In this time period, the pneumatic brake apparatus brakes the web-type recording medium either too weakly or too strongly. Both cases have the result that errors in printing occur, and the print image worsens.
From DE 37 23 284 A1, a pressure regulator is known in which a rotary valve is first adjusted by means of measurement of an actual valve position and comparison with a target valve position, and then controlling the target pressure via the measurement signal of a pressure sensor.
From JP 63-184 116 A, a method is known for adjusting the pressure in a braking apparatus. The pressure of a brake fluid is thereby first set by regulation of the piston position of a valve piston, and then by acquiring the actual pressure, with subsequent regulation.